Add Serializable attribute to suppress CI complaint

Add `Serializable` attribute to suppress CI complaint

Author: peppy

osu.Server.Queues.ScoreStatisticsProcessor/Commands/Score/ChangeModMultiplierCommand.cs

@@ -154,10 +154,11 @@ private void flush(MySqlConnection conn, bool force = false)
         }
 
         [SuppressMessage("ReSharper", "InconsistentNaming")]
+        [Serializable]
         private class ScoreToAdjust
         {
-            public ulong id;
-            public uint total_score;
+            public ulong id { get; set; }
+            public uint total_score { get; set; }
         }
     }
 }